Amyloid and amyloidosis. Amyloid is a generic term referring to a group of diverse, but specific extracellular protein deposits which all have common morphological properties, staining characteristics and x-ray diffraction spectra. Regardless of the nature of the amyloid protein deposited all amyloids have the following characteristics: 1) an amorphous appearance at the light microscopic level and appear eosinophilic using hematoxylin and eosin stains; 2) all stain with congo red and demonstrate a red/green birefringence as viewed under polarized light; 3) all contain a predominant beta-pleated sheet secondary structure; and 4) ultrastructurally amyloid usually consists of non-branching fibrils of indefinite length and with a diameter of 8-12 nm.